A general method for producing a lithium oxide composite includes sequential operations of preparing a transition metal precursor, mixing the transition metal precursor with a lithium compound, and then firing the mixture.
In this course, LiOH and/or Li2O3 are used as the lithium compound. Usually, in the case that a positive active material has Ni content equal to or smaller than 65% of the positive active material, Li2O3 is employed therein. In the case that Ni content is equal to or smaller than 65% of the positive active material, it is preferred to employ LiOH for a low temperature reaction. However, a Ni-rich system, in which Ni content is equal to or larger than 65% of the positive active material, has a problem with a high amount of residual lithium which remains in forms of LiOH and Li2CO3 on the surface of the positive active material due to a low temperature reaction. Such residual lithium, that is, non-reacted LiOH and Li2CO3, reacts with an electrolyte to cause gas generation and a swelling effect, incurring severe degradation of high temperature stability. Furthermore, the non-reacted LiOH may cause gelation due to its high viscosity while mixing slurry before manufacturing electrode plates.
Although a washing process is generally executed to remove the non-reacted Li component, this could still cause surface damage of a positive active material during a washing process, hence degrading the characteristics of capacity and rates and further inducing an additional problem such as an increase of resistance in high temperature storage.